1. Field of the Invention
The present invention relates to a zoom lens system and a camera including the same.
2. Description of the Related Art
In recent years, a camera such as a video camera, a digital still camera, or a silver-halide film using a solid state image pickup element is demanded to have high function and high performance and be small in overall size.
A photographing optical system used in the camera is demanded to be a zoom lens system that has a wide angle of field and high optical performance and is small in overall size.
Further, the photographing optical system is demanded to be a retractable zoom lens system including a configuration for reducing intervals among lens units during non-photographing to intervals different from those in a photographing state and reducing a thickness (length in an optical axis direction) of the entire camera.
Moreover, in a camera employing a solid state image pickup element, various optical members such as a low-pass filter and a color correction filter are arranged between a lens end and the image pickup element. Therefore, when the photographing optical system is used in such a camera, the photographing optical system is requested to be a zoom lens system having long back focus.
Conventionally, as a zoom lens system having a wide angle of field and long back focus, there is known a negative-lead zoom lens system including, in the stated order from an object side to an image side, a lens unit having negative refractive power and a lens unit having positive refractive power (Japanese Patent Application Laid-Open No. H11-52235 (corresponding to U.S. Pat. No. 6,081,389) and Japanese Patent Application Laid-Open No. H06-273670).
As a zoom lens system easily reduced in retractable part and having long back focus, there is known a three-group zoom lens system including, in the stated order from an object side to an image side, a first lens unit having negative refractive power, a second lens unit having positive refractive power, and a third lens unit having positive refractive power (Japanese Patent Application Laid-Open No. 2002-372667 and Japanese Patent Application Laid-Open No. 2004-325975 (corresponding to U.S. Pat. No. 7,050,242)).
Further, there is known a zoom lens system, the entire thickness of which is reduced by using a translucent ceramic material having a higher refractive index than an optical glass (Japanese Patent Application Laid-Open No. 2006-84886 (corresponding to U.S. Pat. No. 7,286,297)).
There is known a zoom lens system having high optical performance with chromatic aberration satisfactorily corrected by using a complex optical element formed by stacking a resin layer on a lens surface of a lens made of a glass material (Japanese Patent Application Laid-Open No. 2004-61519 (corresponding to U.S. Pat. No. 7,027,379)).
In the negative-lead zoom lens system in which the lens unit having negative refractive power is provided closer to the object side, it is easy to increase an angle of field and secure long back focus.
However, in the negative-lead zoom lens system, because the entire lens system is asymmetrical, aberrations, in particular, lateral chromatic aberration often occur.
Further, in the retractable zoom lens system, it is easy to retract the entire system when the zoom lens system is not used and reduce the size of the zoom lens system. However, when the number of lenses of lens units configuring the zoom lens system is large, the length on an optical axis of the lens units increases and, though the zoom lens system is retractable, a retractable part is long.
Therefore, in the retractable zoom lens system, it is important to reduce the number of lenses of the lens units.
In the negative-lead zoom lens system, when it is attempted to reduce the number of lenses of the lens units and reduce the retractable part, aberrations related to imaging performance in a single color such as spherical aberration and coma aberration often occur. It is difficult to correct the aberrations.
In order to obtain high optical performance in the negative-lead zoom lens, it is important to select a lens configuration of the first lens unit having negative refractive power, in particular, a material appropriate for the lenses configuring the first lens unit.
Further, in order to reduce the size of the entire system, it is important to configure the first lens unit, which has an increasing effective diameter, with a small number of lenses.
If the lens configuration of the first lens unit is inappropriate, it is difficult to obtain high optical performance in an entire zoom range at a wide angle of field while realizing a reduction in size of the entire system.